Composition and method for the preparation of deuterated esters of carboxylic acids for gas phase analysis

ABSTRACT

Deuterated derivatives of carboxylic acids, e.g., fatty acids for gas phase analysis, can be prepared by reacting the acid with a deuterated alcohol having up to about 4 carbon atoms in the presence of dimethylformamide dineopentyl acetal.

United States Patent Smith et al.

Jan. 7, 1975 l l l Assignee: Pierce Chemical Company,

Rockford, Ill.

Filed: June 7, 1973 Appl. No.: 367,834

US. Cl. 23/230 M, 23/230 R, 23/232 C, 252/408, 260/410.9 R Int. Cl. G0ln 31/08, Cl 1c 3/08 Field of Search 23/230, 232, 232 C; 260/4109 R; 252/408 References Cited OTHER PUBLICATIONS Thenot et al., Anal. Letters, 5 (4), 217-233 (1972).

Holy, Tetrahedron Letters 7, 585-588 (1972). Buche et al., Agnew Chem. Int. Ed. 3, 62 (I964).

Primary Examiner-R. E. Serwin Attorney, Agent, or FirmWolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[57] ABSTRACT Deuterated derivatives of carboxylic acids, e.g., fatty acids for gas phase analysis, can be prepared by reacting the acid with a deuterated alcohol having up to about 4 carbon atoms in the presence of dimethylformamide dineopentyl acetal.

15 Claims, No Drawings COMPOSITION AND METHOD FOR THE PREPARATION OF DEUTERATED ESTERS OF CARBOXYLIC ACIDS FOR GAS PHASE ANALYSIS The present invention relates to the identification of fatty acids. And, more particularly, to the formation of deuterated esters of fatty acids especially suitable for separation, detection and identification by gas chromatographic mass spectral techniques.

It is well known that natural fatty acids play a key roll in normal metabolism and in an abnormal metabolism responsible for certain circulatory and heart disorders. Accordingly, techniques for easily identifying fatty acids are important in biological and other types of research work.

One method of identifying fatty acids involves esterification of the fatty acids followed by gas phase analysis of the esterified derivatives. Recently Thenot et al. have demonstrated the advantageous analytical application of esterifying fatty acids with dimethylformamide dialkylacetals to form derivatives particularly useful in gas phase analysis. J. P. Thenot et al., ANAL. Letters, 5(4), 217 a 233 (1972). Dimethylformamide dialkyl acetals were first described in the mid-1950's and in the area of analytical chemistry involving fatty acid identification the methyl, ethyl, propyl, n-butyl and t-butyl dialkyls are presently being exploited.

As opposed to the dialkyl acetals identified above, dimethylformamide dineopentyl acetal has not been found to successfully esterify carboxylic acids in the same manner. However, it has been reported that alkylation of mercapto derivatives and esterification of fatty acids with alcohols can be accomplished in the presence of dimethylformamide dineopentyl acetal. A.

Holy, Tetrahedron Letters, 7, 585-588 (1972); G.

Buche et al., Agnew Chem. Int. Ed. 3, 62 (1964). The preparation of esterified fatty acid derivatives by this technique has apparently not become commercially attractiveo Referring still to gas phase analysis of fatty acids, it is recognized that the incorporation of the deuterium atom (heavy hydrogen) in fatty acid derivatives facilitates analysis and identification. For example, volatile derivatives such as deuterated methyl esters of carboxylic acids are especially suitable for inspection by gas chromatographic-mass spectral techniques. Heretofore, however, the preparation of volatile deuterated derivatives of fatty acids for analytical inspection has been difficult and expensive.

A presently practiced method for the preparation of volatile deuterated derivatives involves the use of deuterated diazomethane which, under normal circumstances, will react with carboxylic acids to yield a dideuterated methyl ester. The fact that the deuterated diazomethane is itself toxic and explosive presents obvious disadvantages in respect to its use in preparing deuterated derivatives.

Also, since deuterated diazomethane is an unstable, gaseous substance, it must be independently generated prior to use in special, expensive apparatus to minimize explosion hazards. Deuterated diazomethane can be generated by adding an ether solution of a deuterated precursor to a solution of aqueous ethyl alcohol and potassium hydroxide over a period of time at about 65 C. The deuterated diazomethane can be recovered as an ethereal solution which is distilled from the generating apparatus. The necessity for independently preparing the reactive compound prior to its use in the esterification reaction is, of course, expensive and bothersome.

In addition to the above-mentioned disadvantages with respect to presently available methods for preparing deuterated derivatives of fatty acids, further disadvantages reside in the fact that an ethereal solution is used with the attendant necessity that it be carefully assayed prior to use where accuracy is required, and that reaction with the carboxylic acid is accomplished using an aqueous solution of the acid. This latter aspect requires that, after esterification, extraction with a solvent is necessary to recover the ester for subsequent analysis.

Accordingly, it is a principal object of the present invention to provide an improved method over those now available for the preparation of volatile deuterated derivatives of carboxylic acids for gas phase analysis. Related to this object is the complementary object of providing a safe, efficient and economical technique for preparing deuterated esters of carboxylic acids.

An additional object resides in providing a method for preparing deuterated derivatives for gas phase analysis with the use of available, stable and safe reagents so that the expenses and hazards associated with using explosive and toxic materials are avoided.

A still further object resides in providing a technique for preparing deuterated derivatives of carboxylic acids for gas phase analysis which can be simply accomplished in a single step without the necessity for either carrying out initial preparatory reactions or subsequent recovery operations.

A specific object is to provide a single, stable reactive solution which can be used for the preparation of deuterated derivatives for gas phase analysis in a manner such that the attributes identified in the foregoing objects are realized.

Other objects and advantages of the present invention will become apparent from the following description. And, in this respect, while there will be described herein in detail the preferred embodiments of the present invention, it should be appreciated that the invention is susceptible to various modifications and alternative embodiments. It is to be understood that it is not intended to limit the invention to the specific embodiments disclosed. On the contrary, it is intended to cover all modifications and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. For example, while the invention is specifically illustrated in connection with long chain fatty carboxylic acids, it is applicable to many other carboxylic acids including short chain acids and substituted carboxylic acids such as amide acids.

In accordance with the present invention, the preparation of deuterated esters of carboxylic acids for gas phase analysis can be simply and economically accomplished by reacting under anhydrous conditions the carboxylic acid, or mixtures thereof, to be derivatized with a deuterated alcohol in the presence of N-dimethylformamide dineopentyl acetal. The reaction occurs over a wide temperature range so long as acid solubility is achieved. However, for practical reaction times, e.g. less than about 30 min., a slightly elevated temperature, e.g. 50 65 C., is preferably used. The following example illustrates the present invention.

EXAMPLE 300 microliters of a mixture consisting of 3 pts. by volume of dimethylformamide dineopentyl acetal (CD OD) are 0.5 pts. by volume of completely deuterated methanol are added to milligrams of standard National Heart Institute fatty acid Mixture D in chloroform. Mixture D contains myristic, palmitic, palmitoleic, stearic and oleic acids. The reaction mixture so formed is then gently heated in a closed vial for about 30 min. at about 50 60 C. Aliquots are taken directly from the reaction mixture and introduced into a gas chromatographic column for analysis. The results therefrom indicate that the fatty acids in Mixture D are quantitatively converted to their tri-deutero methyl esters.

While the reaction mechanism involved in the preparation of deuterated derivatives according to the present invention is not completely understood, it is believed that in order to effect substantially complete derivatization of the acids present, at least an equal molar amount of deuterated alcohol must be present. Similarly, at least an equal molar amount of the dineopentyl acetal should be used. A particular advantage of the present invention is that use of the deuterated alcohol or acetal in excess of that required for the reaction is accompanied by no particular disadvantages. Under customary gas analysis conditions, these compounds as well as their reaction by-products are sufficiently volatile with accompanying low retention times such that they do not interfere with the desired analysis.

In further keeping with the advantages of the present invention, it will be appreciated that the mixture of the deuterated alcohol and the dimethylformamide dineopentyl acetal can be preformulated and stored, shipped or the like for use at a future time. So long as the presence of moisture is avoided, the mixture is stable for extended periods of time. Accordingly, storage is usually effected in a closed vessel under nitrogen or similarly anhydrous conditions.

Furthermore, as illustrated in the Example, the reaction with a carboxylic acid or mixtures thereof can conveniently be accomplished in a solvent. It is believed that solvents aid in the solution of the carboxylic acids and thereby enhance the apparent reaction rates. In addition to chloroform, other inert solvents such as dimethylformamide, pyridine, benzene, methylene chloride, THF, as well as other aprotic solvents, can be used. Where the mixture of deuterated alcohol and dineopentyl acetal is preformulated, it may be desirable to include in the formulation an appropriate solvent thereby minimizing the ingredients which the ultimate user need handle in accomplishing the desired reaction.

A particularly desirable feature of the Example illustrated previously is that tri-deuterated methyl esters of carboxylic acids are produced thus permitting greater analytical sensitivity. With the use of deuterated diazomethane, the normal reaction product is a dideuterated ester. However, as will be appreciated, many of the advantages of the present invention are realized with the use of deuterated alcohols other than the completely deuterated specie specifically illustrated though, as a practical matter, there is little incentive for using other alcohols. The principal requirement is that the non-hydroxyl moiety of the alcohol contain at least one deuterium atom and that the alcohol be sufficiently volatile so as not to interfere with gas phase analysis. Among other alcohols useful in the present invention are partially or completely deuterated alcohols having up to about four carbon atoms such as ethanol, propanol, n-butanol, t-butanol, as well as others.

As will be apparent from the foregoing description, the present invention fully satisfies the aims and objectives heretofore recited. The disclosed method is safe, efficient, economical and uses available reagents. As compared with prior methods for the preparation of deuterated derivatives, the method of the present invention requires considerably less labor and has associated with it significantly reduced material and equipment costs.

We claim as our invention:

1. In a process for the gas phase analysis of carboxylic acids comprising forming deuterated derivatives of said acids prior to analysis thereof, the improvement wherein the deuterated derivatives are prepared by reacting the carboxylic acids with a mixture consisting essentially of a deuterated alcohol having up to about four carbon atoms and dimethylformamide dineopentyl acetal.

2. The process of claim 1 wherein a mixture of long chain fatty acids are reacted to form their deuterated derivatives.

3. The process of claim 2 wherein the reaction is accomplished in the presence of an aprotic solvent.

4. The process of claim 3 wherein the deuterated alcohol is partially or completely deuterated methanol.

5. The process of claim 4 wherein the deuterated alcohol is completely deuterated methanol.

6. In a process for the gas phase analysis of carboxylic acids comprising forming deuterated derivatives of said acids prior to analysis thereof, the improvement wherein the deuterated derivatives are prepared by forming a solution comprising the acids, a deuterated alcohol having up to about four carbon atoms and dimethylformamide dineopentyl acetal.

7. A mixture useful for the preparation of deuterated derivatives of carboxylic acids for gas phase analysis comprising a deuterated alcohol having up to about four carbon atoms and dimethylformamide dineopentyl acetal.

8. The mixture of claim 7 wherein the deuterated alcohol is completely deuterated methanol.

9. An anhydrous mixture useful for the preparation of deuterated derivatives of carboxylic acids for gas phase analysis consisting essentially of deuterated methanol and dimethylformamide dineopentyl acetal.

10. The mixture of claim 9 wherein the deuterated alcohol is completely deuterated methanol.

11. The mixture of claim 10 including an aprotic solvent.

12. The process of claim 6 wherein the deuterated derivatives are prepared at about 50 C. 65 C.

13. The process of claim 12 wherein the deuterated alcohol is partially or completely deuterated methanol.

14. The process of claim 13 wherein the deuterated alcohol is completely deuterated methanol.

15. The mixture of claim 8 including an aprotic sol- 

1. IN A PROCESS FOR THE GAS PHASE ANALYSIS OF CARBOXYLIC ACIDS COMPRISING FORMING DEUTERATED DERIVATIVES OF SAID ACIDS PRIOR TO ANALYSIS THEREOF, THE IMPROVEMENT, WHEREIN THE DEUTERATED DERIVATIVES ARE PREPARED BY REACTING THE CARBOXYLIC ACIDS WITH A MIXTURE CONSISTING ESSENTIALLY OF A DEUTERATED ALCOLHOL HAVING UP TO ABOUT FOUR CARBON ATOMS AND DIMETHYLFORMANIDE DINEOPENTYL ACETAL.
 2. The process of claim 1 wherein a mixture of long chain fatty acids are reacted to form their deuterated derivatives.
 3. The process of claim 2 wherein the reaction is accomplished in the presence of an aprotic solvent.
 4. The process of claim 3 wherein the deuterated alcohol is partially or completely deuterated methanol.
 5. The process of claim 4 wherein the deuterated alcohol is completely deuterated methanol.
 6. In a process for the gas phase analysis of carboxylic acids comprising forming deuterated derivatives of said acids prior to analysis thereof, the improvement wherein the deuterated derivatives are prepared by forming a solution comprising the acids, a deuterated alcohol having up to about four carbon atoms and dimethylformamide dineopentyl acetal.
 7. A mixture useful for the preparation of deuterated derivatives of carboxylic acids for gas phase analysis comprising a deuterated alcohol having up to about four carbon atoms and dimethylformamide dineopentyl acetal.
 8. The mixture of claim 7 wherein the deuterated alcohol is completely deuterated methanol.
 9. An anhydrous mixture useful for the preparation of deuterated derivatives of carboxylic acids for gas phase analysis consisting essentially of deuterated methanol and dimethylformamide dineopentyl acetal.
 10. The mixture of claim 9 wherein the deuterated alcohol is completely deuterated methanol.
 11. The mixture of claim 10 including an aprotic solvent.
 12. The process of claim 6 wherein the deuterated derivatives are prepared at about 50* C. - 65* C.
 13. The process of claim 12 wherein the deuterated alcohol is partially or completely deuterated methanol.
 14. The process of claim 13 wherein the deuterated alcohol is completely deuterated methanol.
 15. The mixture of claim 8 including an aprotic solvent. 